The present invention generally relates to semiconductor devices and, more particularly, to a semiconductor device for switching a ballistic flow of carriers with an improved signal-to-noise ratio.
Conventional integrated circuits generally employ bipolar transistors or MOS transistors for signal amplification or switching. Particularly, the high electron mobility transistors (HEMT) and the metal semiconductor field effect transistors (MESFET) that employ the high electron mobility of compound semiconductor materials are characterized by high operational speed and are used in various integrated circuits designed to operate in the microwave frequency range.
In order to fully exploit the excellent high speed performance of these devices, efforts are being made to fabricate the integrated circuits with increased integration density. Such an effort includes the development of submicron patterning techniques for writing a device pattern on a semiconductor wafer with a pattern width that is substantially smaller than 1 .mu.m. With the progress of the submicron patterning technique, semiconductor devices having a size of a few microns or less are now becoming available.
In such submicron devices, although the integration density is increased for increased operational speed, the behavior of electrons in the form of electron waves appears as a strong characteristic and the normal bipolar or MOS operation of the device, that assumes the electrons behave as particles, may be deteriorated, for example by the diffraction or interference of the electron waves.
On the other hand, there is proposed a device called an electron wave device that assumes the wave nature of electrons for the basis of operation. In such electron wave devices, the electron waves are used positively for the device operation and, because of this, the device is free from the problems of excessive miniaturization which affect conventional type semiconductor devices. In fact, the electron wave devices can be miniaturized without theoretical limitation, and such devices are potentially capable of providing the superior performance to that of conventional semiconductor devices with respect to integration density, operational speed, and the like.
FIGS. 1(A)-1(C) show an example of such a quantum interference semiconductor device, as is disclosed in the European Laid-open Patent Application EP-A-0 381 592. This prior art device has a pair of quantum point contacts 10a and 10b for splitting an incident electron wave of a single electron into a pair of electron waves, and the electron waves experience a phase shift upon passage through the respective quantum point contacts 10a and 10b. Upon merging again, the electron waves interfere with each other and produce an output current. The quantum point contact herein means a one-dimensionally confined region acting as a passage of carriers for connecting a first two-dimensional region and a second two-dimensional region with each other. The quantum point contact is confined to have a width such that a number of discrete quantum levels is formed therein and to have a limited length that is approximately equal to or smaller than the elastic or inelastic scattering length of the electrons.
When an electron wave enters into such a quantum point contact, the electron wave experiences a shift in phase in correspondence to the quantum level formed in the quantum point contact. The prior art electron wave device controls the quantum level of the respective quantum point contacts for controlling the mutual phase difference of the electron waves. The device is turned on when the two electron waves undergo a constructive interference and is turned off when the two electron waves undergo destructive interference.
Referring to FIGS. 1(A) and 1(B), this prior art electron device is formed upon a layered semiconductor body 10 in which a two-dimensional electron gas is formed. The layered semiconductor body 10 includes an n.sup.+ -type AlGaAs doped layer 11 for supplying electrons and an undoped GaAs channel layer 13, with an intervening undoped AlGaAs spacer layer 12 as usual. In this structure, the foregoing two-dimensional electron gas is formed along an upper boundary of the channel layer 13 in correspondence to the heterojunction interface between the AlGaAs layer 12 having a large band gap and the GaAs layer 13 having a smaller band gap, as is well known in the HEMT structure.
Similar to the usual HEMT, there is provided a source electrode 15 and a drain electrode 16 on the doped AlGaAs layer 11 with an ohmic contact to the layer 11, and the source electrode 15 injects the electrons into the two-dimensional electron gas while the drain electrode 16 recovers the electrons from the two-dimensional electron gas. In the region of the layer 11 formed between the source electrode 15 and the drain electrode 16, there are provided Schottky electrodes 17 and 18 to interrupt the flow of electrons from the source electrode 15 to the drain electrode 16 such that the Schottky electrode 17 is separated from the Schottky electrode 18 by a gap, and there is provided another Schottky electrode 25 in correspondence to the gap between the electrodes 17 and 18.
In this structure, it will be understood that there are formed a pair of electron passages, or passageways, one between the electrode 17 and the electrode 25, and the other between the electrode 18 and the electrode 25. These passages 24, however, are confined laterally when viewed in the direction of the flow of electrons, as shown in FIG. 1(B), due to the depletion regions 21, 22 and 23 formed under the electrodes 17, 18 and 25. Thereby, there appear discrete quantum levels in each of the passages 24 when the width of the passage is confined to a dimension substantially smaller than the de Broglie length of the electron waves. Further, each of the electrodes 17, 18 and 25 has a size in the propagating ,direction of the electron waves typically, of 1-2 .mu.m or less, and thus smaller than any of the elastic and inelastic scattering lengths of the electrons. Thereby, it will be understood that each of these passages 24 forms a quantum point contact as defined before.
When a single electron is injected by the source electrode 15, the electron, represented by the electron wave W0 in FIG. 1(C), passes through the quantum point contacts as the split pair of electron waves W1 and W2, wherein the electron waves W1 and W2 represent the probability of the electron passing the respective channels. Upon passage through the quantum point contacts, the electron waves W1 and W2 merge with each other and undergo the interference as described previously.
FIG.2 shows another example of the conventional quantum semiconductor device disclosed in the European Laid-open Patent Application EP-A-0 461 867 wherein a two-dimensional electron gas 30 is formed along a heterojunction interface, similarly to the previous device. Schottky barriers 32a and 32b are formed adjacent to an emitter 31 with a quantum point contact 30a formed therebetween, and an electron wave is radiated from the quantum point contact 30a with a directivity determined by the quantum state of the electron wave as indicated by an angle .theta., upon injection of the electrons from the emitter 31.
The electron wave thus radiated experiences a refraction by a potential that is induced by a control electrode (not illustrated) similarly to an optical wave and is detected by another quantum point contact 30b that is formed between a pair of Schottky barriers 34a and 34b. When the electron wave hits the quantum point contact 30b, an output voltage is detected by a collector 35.
Further, Japanese Laid-open Patent Publication 3-91961 corresponding to the U.S. patent application Ser. No. 400,416 by Heiblum describes an electron wave device that uses a potential prism for refracting and reflecting an incident electron wave. There, the device injects electron waves via one or more quantum point contacts, and the refracted or reflected electron waves are detected by a plurality of collectors that are provided also in the form of quantum point contacts.
In such conventional quantum semiconductor devices, it has been noticed that the output voltage obtained at the collector is generally very small. The reason of this undesirable effect is attributed to the existence of the two-dimensional electron gas also in the region located below the collector electrode. More specifically, any voltage change induced in the collector in response to the detection of the electron wave tends to be canceled out by a flow of electrons that occurs in the two-dimensional electron gas so as to compensate for any change in the potential. In order to detect such a very small output voltage, an amplifier having a very large input impedance such as a lock-in amplifier has to be used. However, such an amplifier has a complex construction and use of the quantum semiconductor device in practical applications has been discouraged. In the device of Heiblum that includes a plurality of collectors, too, such a counter-flow of carriers occurring in the two-dimensional electron gas obscures the switching operation of the electron wave between different collectors. Thus, conventional quantum semiconductor devices have suffered from the problem of small logic amplitude. Further, the voltage applied to the collector region tends to affect the refraction or interference of the electron waves.
In addition, the conventional device described previously has a problem of large capacitance between the control electrode used for inducing the potential 33 and the electrons included in the two-dimensional electron gas. Thereby, a very large control voltage and electric power has been needed for controlling the flow of electrons to obtain a very small output voltage. Further, the conventional device that uses the two-dimensional electron gas has another drawback in that the injected electron waves tends to experience scattering by the electrons excited thermally in the two-dimensional electron gas. Thus, the conventional devices exhibit the switching operation only at very low temperatures in the order of 1K. Even when the electron wave is injected in the form of hot electrons, scattering of the electrons in the two-dimensional electron gas is inevitable due to the electron-electron interaction.
Summarizing the above, conventional quantum semiconductor devices that use the quantum mechanical carrier wave have been extremely vulnerable to noise and could be operated only under an extremely low temperature environment.